Calibration
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Calibration is the determination, by measurement or comparison with a standard, of the correct value of each reading on a measuring instrument. The standard may be maintained by a national or international organization.
For physical constants, weights, and measures, there are known and agreed values in the International System of Units (SI). Such constants include the length of the metre, the mass of the kilogram, and the volume of a litre.
In the USA, the National Institute of Standards and Technology, a part of the federal government, maintains standards and is considered the arbiter and ultimate (in the U.S.) authority for values of SI units and industrial standards. NIST also defines traceability, by which an instrument's accuracy is established in an unbroken chain relating an instrument's measurements through one or more derivative standards to a standard maintained by NIST, as well as uncertainty.